Typical high performance amplifiers use either straight class A, active bias class A, or type AB output stages. The high quiescent idling power of conventional class A amplifiers requires expensive, high capacity cooling provisions for the output stages. Active biasing schemes are available wherein the average signal level sets the class A idling current to reduce the heat dissipation requirements of high power, class A systems. In many instances, the class AB systems are desirable because the steady state dissipation is kept to a reasonably low fraction of the rated output powder thereby reducing the heat dissipation requirement.
An important consideration in the design of either class A or class AB output stages is the thermal-runnaway effect associated with bipolar transistors. As is well known, the base-emitter voltage drop of a bipolar transistor decreases with temperature (-2.4 mV/.degree. C.) thereby increasing the collector-emitter current through the device. Naturally, the self-heating in the transistor increases with increased current flow, which, in turn, further decreases the base-emitter voltage drop, leading to thermal-runnaway. Consequently, without adequate heat dissipation means, such as an adequate heat sink, or independent negative thermally induced feedback which could act as a thermal protection circuit, such output stages are notoriously known to self-destruct when driven to high output levels at high ambient temperatures. Active current limiting circuits have been utilized to overcome this problem but they are, in general, complex and compromise the peak power performance of the amplifier. The problems with thermal instability have been reduced in amplifier designs by using metal oxide semiconductor-field effect transistors (MOS-FET) which do not exhibit the negative temperature coefficient of the bipolar transistors. In fact, a MOS-FET exhibits a positive temperature coefficient which tends to self-limit the maximum temperature and current. The problems associated with the MOS-FET is they have heretofore been utilized in a manner requiring a significant voltage drop across the MOS-FET which, in turn, translates into a relatively high heat dissipation.